1. Field of the Invention
The present invention relates to a ball joint and, more particularly, to a ball joint for use in the steering mechanism or suspension mechanism of automobiles.
2. Description of the Prior Art
Generally, the ball joint for the above-mentioned uses has a socket which receives the head portion of a ball stud through the medium of a bearing member.
In the known ball joint of the type mentioned above, when a plastic is used as the material of the bearing member, a problem is caused due to a large difference in the coefficient of thermal expansion between the plastic and the metal used as the material of the socket and the ball stud. For instance, a polyester resin exhibits a coefficient of thermal expansion which is about 17.times.10.sup.-5 /.degree.C. to 18.times.10.sup.-5 /.degree.C., while iron exhibits a coefficient of thermal expansion generally ranging between 1.16.times.10.sup.-5 /.degree.C. and 1.2.times.10.sup.-5 /.degree.C. Therefore, in the ball joint in which the bearing member is fitted with a high pressure into the socket, the friction torque is increased as the bearing temperature rises. In addition, the friction torque itself has a large absolute value, resulting in a large static friction torque when the joint starts to operate. This increased static friction torque appears, for example, as an increased manipulating force for manipulating the steering wheel.